1. Field of the Invention
This invention is directed to a drive mechanism for accurately positioning a work stage along two axes with three degrees of freedom.
2. Related Art
Mechanisms for positioning a work stage for locating a work piece are well known in the art. For example, U.S. Pat. No. 4,528,490 of Hansen for xe2x80x9cTwo Axis Drive for Stagexe2x80x9d includes a base plate and an additional pair of stacked plates, one known as a stage plate and the other as an intermediate plate. Each plate is driven by a drive. The intermediate plate is driven by drive bar along a linear direction with respect to the base plate. A drive means for the stage plate includes a motor driven capstan, and a drive bar has angular freedom of movement, i.e. is pivotally mounted under stage plate. The stage plate is thus free to move along a second path and to rotate.
J. Reed et al xe2x80x9cHigh Speed Precision X-Y Stagexe2x80x9d, J. Vac. Sci. Technol. B, Vol. 3, No. 1 pp. 112 et seq., (Jan./Feb. 1985) describe conventional linear ways and ball bearings but xe2x80x9cincorporates a unique capstan/swinging drive bar design to couple the servomotors and the X and Y stage elements.xe2x80x9d The stage comprises three stacked plates coupled to one another by crossed linear bearings. The bottom plate is affixed to a base. The center plate is fixed rigidly to its drive bar through a preloaded duplex bearing. The drive bars are driven by a servomotor via a friction-drive capstan.
S. Ido et al. xe2x80x9cPrecision X-Y Stage for Electron Beam Lithography Systemxe2x80x9d pp 267-268 Bull. Japan Soc. of Prec. Eng. Vol. 18, No. 3 (September 1984) describe a stacked X-Y stage configuration with hydrostatic bearings.
Kallmayer et al xe2x80x9cX-Y Tablexe2x80x9d IBM Technical Disclosure Bulletin Vol. 30, No. 7 (December 1987), pp. 376-377 show three rigidly mounted spindle drives with stators affixed to a rigid support so they do not pivot relative to the table 4, and so the flexibility of the drives is limited by their rigid mounting to a restricted range of motions afforded by the guides and in the slots in the table 4. In addition, two of the spindle drives are parallel to each other. Laser interferometers and mirrors are used to measure displacement.
Tsuyuzaki et al. J. Vac. Sci. Technol. B, Vol. 4, No. 1 p28X, (January/February 1986) describe a plate structure or xe2x80x9cplanarxe2x80x9d stage with X-Y motion accuracy imparted via machined guide slots in the base and a substrate positioning table. An X-Y cross structure is placed within these slots between the top table and the base. A low friction polymeric material such as PTFE is employed as a bearing surface.
Constant Download Friction Drive System, IBM Technical Disclosure Bulletin, Vol. 32, No. 8A, (January 1990) pages 120-121 describes a method and apparatus for canceling the variation in download exerted by the end of a drivebar system by preloading the drivebar as shown in FIG. 3 herein.
Ball type pivot joints provide multiple degrees of freedom about a point at the center of a ball of spherical shape. Heretofore, ball joints have included two basic types including as follows:
(1) Spring-loaded joints and
(2) Spherical bearings.
Spring-loaded ball joints include a pair of cones or sockets which are tightened against the ball to eliminate backlash. A disadvantage of that system is that when using high preloads for high linear stiffness, there is considerable friction and wear between the ball and the cones or sockets.
The spherical bearing ball joint relies on closely matched inner and outer spherical bearing elements. This type of ball joint has high linear stiffness and low friction, but by the nature of this design must operate with a small clearance between the inner and outer bearings. Such a small clearance increases with wear and the backlash associated with this clearance makes it unsuitable for micropositioning applications.
U.S. Pat. No. 5,140,242 to Doran et al., which is herein incorporated by reference in its entirety, discloses a servo guided stage system having integrated dual axis plane mirror interferometers for sensing stage position. U.S. Pat. No. 5,052,844 to Kendall, which is herein incorporated by reference in its entirety, discloses a ball joint with an adjustable preload for use in a stage positioning system.
Conventional methods for positioning a substrate, however, use costly or ineffective means to measure the yaw angle of the stage. What is needed in the art is a stage position system having simplified yaw angle measuring device.
Now, according to the present invention, the above-described and other disadvantages of the prior art are overcome or alleviated by the positioning stage comprising a base having rectilinearly disposed x and y axes, a stage plate slideably supported on said base, at least three linear drive means rotatably engaging both said stage plate and rotatably engaging said base for moving said stage plate on a path with at least three separate drive displacements along said rectilinearly disposed x and y axes and rotation of said plate on said base substantially parallel to the surface of said base, said plate moving upon the surface of said base, whereby combined x, y rectilinear and rotary motions with respect to said base can be achieved, an x interferometer and receiver for determining x position, a y interferometer and receiver for determining y position, and, a yaw sensor device for determining yaw angle comprising a light emitting source, a cylinder lens, and a position sensing detector. In this manner, the x position measurement, the y position measurement, and the yaw measurement can be used to close mechanical servo loops to accurately position the stage plate and eliminate yaw errors.
A method for determining yaw angle of a positioning stage also is provided comprising directing an incident beam of light from a light emitting source at a reflective surface on said positioning stage to create a reflected beam of light, passing said reflected beam of light through a cylinder lens to vertically focus said reflected beam of light on a position sensing detector, and generating a signal from said position sensing detector, wherein said signal is dependent upon the lateral position of said reflected beam on said position sensing detector.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.